A CLEAN SHAVE

Gustavus Adolphus, Albrecht von Wallenstein

Wounded Cavalier and stern Puritan

Various eighteenth-century gentlemen.

In the seventeenth century, wearing a goatee with an impressive moustache was fashionable.
After the war, the beard completely vanished from the picture, and a thinner moustache would remain until the first decades of the next century (when Louis XIV noticed silver hairs on his upper lip, the latter trend was established).
Eager to Westernize his old-fashioned realm, Czar Peter the Great demanded that all the nobility in Russia should be clean shaven (the Orthodox clergy, however, was allowed to keep its white beards as a symbol of authority). The Czar alone displayed a fashionable dark mo (the rights of royalty, eh?).
There are several theories around about what had triggered the clean shave trend...

i) Puritans. After taking over the British Isles. Both in uniform and in civvies, Cromwell's adult male followers were always clean shaven. They shaved themselves clean for no facial hair to taint the host and the wine that are the flesh and blood of Christ during the Eucharist.

ii) Snuff. After the 30YW, powdered tobacco replaced pipes as the form of nicotine consumption in vogue, and facial hair would get all dirty should snuff would fall into it.

iii) Keeping mess out of the face, in general. This is a more general application of the snuff hypothesis above, relating to any kind of mess and to the now more polished courtly manners.

iv) Public security. Like the foot-length cloak (replaced with short capes) and the cavalier hat (replaced with the tricorn), the moustache and beard could as well conceal a criminal. A lighter kind of apparel made it easier to recognize a person as not suspicious or shifty.

v) To look more childish and cuter. The whole post-30YW courtly trend, with its pastel colours, its love of sweets washed down with chocolate and fruit liquor, its love for frills and lace, its gardens whose plant life was bustling with bright flowers... is generally light and airy, oriented towards cuteness, as a manner of finishing with a past that had been too dark and grim.

Most of these theories, except the Puritan one, stem from Wolfgang Schivelbusch's 

Tastes of Paradise: A Social History of Spices, Stimulants, and Intoxicants.